Fuel gauge not reading correctly

Fuel mileage is not calculated off the level in the tank. That would be horribly inaccurate because the gas sloshes around over bumps and when cornering. Instead, the engine computer knows what fuel pressure is supposed to be, and it very carefully calculates exactly how long to pulse the injectors open. With all the computer knows, it knows how much fuel is going into the engine. That information is shared with the body computer and the instrument cluster. The way to approach this is to use a regular scanner to view live data and see what the instrument cluster is seeing for fuel level. If that display shows it going close to "empty", we would know the float and sending unit are working properly and the problem is in the cluster. More commonly, I suspect the float is hanging up on a baffle inside the tank. If that is the case, the scanner will show the level as "3/4" all the time, and the dash gauge is just responding appropriately to what it is being told.

If the gauge drops to "E" when you turn the car off, then the problem is in the fuel pump (common on Blazers and Jimmy's). If it does not drop to "E" when you turn the car off, then the problem is in your instrument cluster (not as common, but not unheard of either). Let me know what it does and I will go from there.